Titans

Titans are the enemy in Revenge of the Titans and the alien invasionary force attacking Earth. The primary objective in the game is to kill all advancing Titans before they reach your base.

You can read the basics of Titan behavior.
A note on Endless mode : Titan genus appear on endless mode if one of two possibles conditions are reached : either having a particular technology or being at an advanced enough level will make them appear. It allows you to manipulate somewhat their appearance. Some informations about Titans spawn points you can read in Titan basics.

If you need explanation to better understand this table, the Titan statistics page should answer to your questions. There is also the Armor page, because armor is both a complicated and important subject. Understanding how threat from your weapons can influence their path can be also useful.